What Is Fast Cache In Vnx?

Quick Fix Summary

What’s Happening: FAST Cache in VNX Explained

FAST Cache in EMC VNX is basically a secondary cache that runs on SAS Flash drives to speed up access to the data you use most. Unlike DRAM cache—which is fast but limited in size and disappears when the system shuts down—FAST Cache uses non-volatile flash storage to extend caching way beyond what DRAM can handle. It automatically moves the hottest data (the stuff getting hammered with I/O requests) from slow HDDs to blazing-fast flash drives, cutting latency and boosting response times without you lifting a finger.

Think of it as teamwork: FAST Cache works with DRAM cache (the primary, super-fast but small buffer) and FAST VP (which shuffles data between drive tiers based on how often it’s used). FAST VP is more of a long-term mover—it relocates data between SAS and Flash drives over time. FAST Cache, though, is the real-time bodyguard for your most active data. Together, they handle mixed workloads—like databases or virtualized setups—like a dream.

Step-by-Step: Enabling FAST Cache in VNX

Here’s how to turn on FAST Cache if you’re running VNX OE for Block 05.33.009.7.119 or later (as of 2026). Older versions might not play nice with newer flash drives, so don’t skip this check.

1. Fire up Unisphere: Open Unisphere (version 1.6 or later) and log in with admin rights.
2. Find Your Pool: Head to Storage ➔ Pools and pick the pool where you want FAST Cache (for example, Pool_0).
3. Open Pool Settings: Right-click the pool and hit Properties. Or just click the Properties icon up top—whichever’s easier.
4. Flip the FAST Cache Switch: In the FAST Cache tab, check Enable FAST Cache. If the option’s grayed out, your pool might not support it (all-flash pools, for instance, rely solely on DRAM cache).
5. Pick Your Cache Drives: Select the SAS Flash drives you want to use (say, 400GB SAS Flash 2 drives). Mixing sizes works, but Dell EMC suggests sticking to the same capacity for consistency.
6. Set the Cache Size: Decide how big you want the cache (for example, 1TB). The max is usually 10% of the pool’s total capacity—adjust the slider or type in a value.
7. Save and Wait: Click Apply to lock it in. The system will spin up the cache, which could take a few minutes depending on how many drives you’ve got and their sizes.
8. Check the Dashboard: Make sure FAST Cache is live by peeking at Dashboard ➔ Performance. Look for lower I/O latency and higher cache hit ratios (aim for over 90% to really see the benefits).

If This Didn’t Work: Troubleshooting FAST Cache

FAST Cache not giving you the performance boost you expected? Or maybe it won’t even turn on? Here’s where to look:

• Check Drive Support: Not all flash drives play ball. Peek at the VNX Drive Compatibility Matrix (2026) to confirm your drives are approved. Consumer-grade SSDs? They’re out.
• Confirm Pool Type: FAST Cache only works with hybrid pools (mix of SAS and Flash drives) or SAS-only pools. All-flash pools? They skip FAST Cache entirely and rely on DRAM cache instead.
• Run a Health Check: Head to Settings ➔ Diagnostics ➔ Health Check to spot issues. Common culprits include:
  • Not enough cache drives: Add more SAS Flash drives to the pool.
  • Failed drives: Swap out any busted flash drives in the pool.
  • Outdated firmware: Bump your VNX OE for Block up to the latest version (minimum 05.33.009.7.119).

Prevention Tips: Keeping FAST Cache Running Smoothly

Don’t wait for problems to pop up—keep FAST Cache humming along with these habits:

• Watch the Cache Hit Ratio: Use Unisphere or naviseccli to track how often your cache is actually hitting the mark. Below 80%? Your cache might be too small or your data promotion settings too aggressive. Tweak the size or review your workload patterns.
• Keep Firmware Fresh: Dell EMC rolls out updates to improve FAST Cache’s smarts and compatibility. Schedule updates during downtime to avoid headaches.
• Spread Out the Hot Data: Don’t let a few LUNs hog all the cache. Use naviseccli storagepool -list to see how your pool’s being used and rebalance if things look uneven.
• Replace Worn-Out Drives: SAS Flash drives wear down over time (usually 3–5 years). Keep an eye on drive health in Dashboard ➔ Hardware ➔ Drives and swap out drives showing high wear.
• Log Your Changes: Jot down every tweak you make to FAST Cache—like which drives you picked and how big you set the cache. It’ll save you a ton of time if performance issues pop up later.